One of fields expected for application of a wide bandwidth light source is clinical medicine. Conventionally, for finding early diseases such as a cancer, it has been widely carried out to obtain a tomographic image of a specific site of a human body by using a variety of approaches such as X-ray, sonic wave, or MRI (Magnetic Resonance Imaging tomography.) Such the tomographic image with higher resolution makes higher or more reliable pathologic diagnosis possible. Thus, as exemplified by OCT (Optical Coherence Tomography,) development has been continuing for obtaining the image with high spatial resolution, enabling to distinguish each cell. For this purpose, a broadband light source is desired in order to radiate light, which is emitted from an interferometer to a tissue.
Typical approaches for obtaining the wide bandwidth light source include the following examples.    (1) Approach by using an active gain medium such as a laser medium.    (2) Approach by using nonlinear effects
Of these two approaches, the approach using the active gain medium such as a laser medium to increase in the gain is described as follows. In this approach, the wide bandwidth light source is realized by amplifying the spontaneous emission light (ASE: a noun expressing a sense opposite to stimulated emission light). Such the wide bandwidth light source includes an optical fiber (EDFA: Er-doped fiber amplification, etc.) doped with rare earths ions, for example, Er (erbium,) Tm (thulium,) Pr (praseodymium,) and the like (refer to patent document 1.) According to patent document 1, a pump light having different wavelengths or light intensities is coupled to each of both ends or near places around the both ends of the Er-doped optical fiber. Then, the spontaneous emission light, which is emitted by absorption of the pump light in a rare earth ions contained in a core of rare earth-doped fiber, is used as an output of the wide bandwidth light source.
Next, (2) Approach by using the nonlinear effect is described as follows. As will be described below, this approach utilizes nonlinear effects in fibers such as. super-continuum(SC) generation, optical solitons, and so on. The super-continuum light source generates the light with an ultra-wide bandwidth by launching a high intensity light pulse into the third-order nonlinear medium. In addition, it has been proposed to make an optical soliton pulse as the wide bandwidth white light by using adiabatic pulse compression device composed of an anomalous dispersion optical fiber and the white light-emitting device, which is composed of a normal dispersion optical fiber and used for expanding a spectral width (refer to patent document 2.) According to this proposal, the light pulse, which is outputted from a light pulse source, is amplified up to a peak intensity, in which an optical soliton of the first-order is made at the emission end of a adiabatic pulse compressor, by using a light amplifier. Subsequently, by using the adiabatic pulse compressor, as the optical fiber, in which an anomalous dispersion value decreases from an incident end to the emission end, the optical soliton is subjected to adiabatic compression. The optical soliton compressed in such the way is properly amplified by another light amplifier. Then, a spectral width is expanded by using the white light emitter to emit as the white light having a high coherence and the wide bandwidth. In this way, a coherent light source is obtained providing the wide bandwidth of 225 nm of which wavelengths range from 1.45 μm to 1.68 μm.
For the optical coherence tomography as described first, a proposal has been made to measure information contained in a living body tissue by using a superluminescent diode (SLD) as the light source through optical coherence (interference fringe) (refer to non-patent document 1, for example.) The SLD light source has short temporal coherence and high spatial coherence unlike a laser sources. Where, the temporal coherence is defined as the correlation or predictable relationship of phase between optical waves observed at different moments in time. The spatial coherence is defined as the correlation of phase between optical waves at different points in space. In the case of the superluminescent diode (SLD) light source having 16 nm of the spectral width, the spatial resolution ranges from 10 to 20 μm. The spectral width is in the inverse proportion to the spatial resolution. Therefore, in order to obtain a high spatial resolution, the spectral width should be expanded.
There are the following problems in the conventional approaches as described above.    (1) The case of the wide bandwidth light source using active ions of rare earths arises a problem in that the bandwidth of the wide bandwidth light source is limited to a gain spectrum bandwidth of the active ions.    (2) The case of the wide bandwidth light source using the optical soliton pulse arises a problem in that (a) modulation instability is present (side-band instability.) In addition, there is the problem of (b) nonlinearity and dispersion controls are difficult. Particularly, to solve the latter (b) problem, dispersion should be closed to zero as possible to produce short pulses. However, there arises the following problem in this way: stable manufacture is difficultly realized and, also, reproducibility becomes difficult.
Therefore, the proposal is made for the light source providing a strong optical signal and a very wide bandwidth in comparison with these conventional techniques. According to this proposal, the laser light source called a noise-like laser is used as an optical pulse-generating device. Moreover, accompanied with this, a highly nonlinear optical fiber such as a Ge(germanium) doped Silica based optical fiber and a photonic crystal fiber is used as device for generating the white pulses as a wide bandwidth light. The noise-like laser is described below. The wide bandwidth light source according to this proposal has an optical signal spectrum with excellent flatness of light intensity ranging from 1.2 μm to 2.0 μm wavelengths. Thus, using this wide bandwidth light source enables to perform loss evaluation of a wide range of optical components, which includes that used for optical access systems, for the range of wavelengths between 1.2 μm and 1.65 μm. In addition, use of the wide bandwidth light source as the light source for OCT makes possible very excellent precision analysis carried out in some micrometers or shorter spatial resolution.
FIG. 1 shows an example of a conventionally proposed wide bandwidth light source using the laser light source called the noise-like laser. Wide bandwidth light source 100 is constituted by optical pulse-generating device 102 comprising ring resonator fiber laser 101 and white light-emitting device 104 for inputting optical pulse 103 outputted from this optical pulse-generating device 102. White light-emitting device 104 is constituted by HNL (highly nonlinear) fiber 105. Where, in HNL fiber 105, a dispersion value is −0.60 ps2/km per 1550 nm, zero dispersion wavelength is 1532 nm, dispersion slope is −0.0366 ps2/km/nm, nonlinear constant is 20/W/km, loss is 1.59 dB per 1550 nm, and length of fiber is 1 km. Optical pulse-generating device 102 is connected to optical pulse-emitting device 104 by using single mode optical fiber 106.
FIG. 2 shows a configuration of the optical pulse-generating device according to this proposal. Optical pulse-generating device 102 has ring resonator fiber laser 101. In ring resonator fiber laser 101, one end of each of a first and second single mode optical fiber (SMF) 113 and 114 made of Coming Incorporated. (USA)-made Flexcore(product name is “Flexcore 1060”) is connected to each of one end of a first and a second dispersion shift fibers (DSF) 111 and 112 having 1.8 m each the length and Er-doped optical fiber (EDF) 115 having 2.5 m length is connected to the other ends of the above fibers. To Er-doped optical fiber (EDF) 115 is connected WDM (wavelength division multiplexing) coupler 118 for inputting pump light 117 of 1480 nm wavelength from laser diode (LD) 116 as pump light source for pumping.
On each other side of first and second dispersion shift fibers 111 and 112, a first or a second collimate lenses 121 and 122 are arranged to convert the light to be propagated to a spatial parallel beam. In space portions opposite to these first and second collimate lenses 121 and 122, first λ/4 waveplate 123, the λ/2 waveplate 124, polarizing beam splitter (PBS) 125, optical isolator (ISO) 126, and second λ/4 waveplate 127 are arranged serially starting from the first collimate lens 121. The laser light split by polarizing beam splitter 125 is coupled to output optical fiber 129 by using output coupling collimate lens 128 to be sent to an output port not illustrated. The total length of ring resonator fiber laser 101 is 14 m and a longitudinal mode interval of the optical resonator is 14.3 MHz.
In this conventional optical pulse-generating device 102, optical fiber 106 is used for ring resonator fiber laser 101. Optical isolator 126 propagates a light wave traveling in the right direction and blocks the light wave traveling in the opposite direction in FIG. 2. Therefore, the light pulse circulates clockwise around inside of the ring resonator. The output light thereof is partially extracted from the output port by an output coupler constituted by λ/2 waveplate 124 and polarizing beam splitter 125, rotating λ/2 waveplate 124 causes rotation of a plane of polarization. On the other hand, polarizing beam splitter 125 has characteristics in that a horizontally polarized light travels straight and a vertically polarized light is reflected on a 45 deg plane. Using these characteristics allows an output ratio of the output coupler to be variably regulated in the range between 0 and 100 percents.
Meanwhile, first λ/4 waveplate 123 is used for adjustment of the polarization state, which is normally elliptic polarization, of the light wave inputted from first dispersion shift fiber 111 to a desired polarization state. Second λ/4 waveplate 127 is used for adjustment of the polarization state, which is linear polarization, of the light wave inputted passing through optical isolator 126 to make it coincide with an inherent polarization mode state of an optical fiber path.
FIG. 3 shows a temporal waveform of an output pulse train outputted by this conventional optical pulse-generating device. Optical pulse 103 (FIG. 1) outputted by optical pulse-generating device 102 illustrated in FIG. 1 or FIG. 2 makes a pulse train generated in a predetermined time interval. The time interval of optical pulse 103 is 70 ns nanoseconds: 10−9 seconds) and the cyclic frequency is 14.3 MHz.
FIG. 4 shows an example of a spectrum of the optical pulse outputted by the conventional optical pulse-generating device. A full width at half maximum of the spectrum of optical pulse 103 is 87 nm.
FIG. 5 shows a change of the spectrum with an average output power of the optical pulse-generating device and the length of the single mode optical fiber. The same fig (a) coincides with the spectrum of the optical pulse shown in FIG. 4. In FIG. 5, the combination of the average output power (mW) of optical pulse-generating device 102 (FIG. 1) with the length (m) of the single mode optical fiber 106, in which optical pulse-generating device 102 shown in FIG. 1 is connected to optical pulse-emitting device 104, is changed. From these figures, it can be known that the optical pulse with a relatively narrow bandwidth as shown in the same figure (a) changes to the optical pulse with a relatively wide bandwidth as shown in the same figure (e.)
As described above, realizing wide bandwidth light source 100, shown in FIG. 1, by using the noise-like laser enables to realize the wide bandwidth light source having a flat and preferable spectrum characteristic in comparison with the conventional wide bandwidth light source. For example, optical pulse 103e as shown in the same figure (e) realizes, in the case where an average input power is 72 mW, the flat and preferable spectrum characteristic having a bandwidth of 800 or more nm in the range between 1200 nm wavelength and 2000 nm wavelength.
Patent document 1: Japanese Published Unexamined Patent Application No. 2003-347630 (paragraph No. 0011, FIG. 1)
Patent document 2: Japanese Published Unexamined Patent Application No. H11-160744 (paragraph No. 0037, FIG. 3)
Non-patent document 1: Optronics Monthly (July 2003, p. 219)
However, the wide bandwidth light source according to the present proposal uses passive mode-locking. Therefore, when vibration and a temperature change occur in outside of the wide bandwidth light source, a signal spectrum and an output light amount vary sensitively and, thus, mode-locking is not stably reproduced. Hence, in the worst case, mode-locking is not maintained. Moreover, in the noise-like laser as the optical pulse-generating device at the start of the operation, a light ray propagation path thereof is partially constituted by the optical fiber. Therefore, when a temperature as an external environmental factor changes, a thermal stress of the optical fiber constituting the noise-like laser changes. As the result, an internal polarization state of the optical fiber is changed by the stress and a temperature status of the optical fiber at different times. Consequently, a very small birefringent amount of inside of the optical fiber changes and, then, the inherent polarization mode state for internal propagation is broken. From these causes, there is a problem in that mode-locking is difficult to operate in the wide bandwidth light source according to the present proposal.
When stable operation is not performed, output stability of the wide bandwidth light source according to the present proposal is 1 or more dB in the case where measurement is carried out for 1 hour. For use as the light source for loss evaluation of optical components, output stability should be 0.3 or less dB and preferably 0.1 or less dB. Therefore, a stable operation method providing a less spectrum variation should be found. Also for external factorial changes such as temperature change, a stable operation mechanism to maintain the polarization mode state of the optical fiber and keep optical output strength stable is absolutely necessary.